Directional transmission line transducer



July 7, 1953 NORTON 2,644,928

nmsc'rxomu. TRANSMISSION LINE mmsoucsa Filed June 9, 1948' 2 Sheet-Sheet2 I III/ 1111 v i i 'IIIIIIIIIIIl/IIII/lIlIIlIIIIllII/Il ATTORNEYPatented July 7, 1953 DIRECTIONAL n sMl sl n LINE 'rn NsnUcun Lowell E.Norton, Princeton Junction, 'N.J.',- as,-. si ner to R d o (inven io ofAmer a. a an rot on 9. De awar Application June 9, 1948, Serial No.31,850

, 8 Claims. (01.333-33) This invention relates to directional trans- Iducers, and more particularly to directional transducers for use withtransmission lines.

When speaking of transmission lines herein,

the term is used generically to denote an elec- I trical line for theconductienof electromagnetic energy, so that the term includes hollowpipe waveguides, as well as two-conductor transmission lines such ascoaxial transmission lines or coaxial waveguides, and open wiretransmission lines. In using transmission lines for the conduction ofenergy from a source or generator to a load, it is ordinarily desirablethat the load presented to the generator shall be substantially free 01changes due to changes in the load at the far end of the line. This isparticularly true with magnetron generators for example. It is wellknown that magnetron generators are particularly sensitive to changes inload which cause the generator to change the frequency of the generatedwaves, or to increase or decrease the amplitude of the generated waves.Thus, the phenomenon of puIling in connection with magnetrons is wellknown to the art. This problem becomes particularly acute when the finalor useful load,'such as an antenna, to which the power from the'generator'is fed, is physically located some distance from thegenerator. along a transmission line whichis many wavelengths long whenhigh frequencies are employed. As a result slight changes in theantcnnaioading are magnified due to the long electrical length of theline. The magnified change in load presented to the generator may beminimized by placin i t ine a directienel t a uce A- The power isusually fed tothe antenna" It is another object of the invention toprovide a directional transducer relatively insensitive to load changes.

These and other objects, advantages, and novel ieatures of the inventionwill be more clearly understood from the following description taken inconnection with the accompanying drawing in which:

Figure 1 is a schematic diagram of an open wire transmission linebetween a high frequency generator and a load and employing thedirectional transducer of the invention;

Figure 2 is a face view of a directional transducer of the invention asused in a rectangular waveguide transmissionline;

Figure 3 is a face view of a'directional transducer of the inventionusing circular waveguide;

directional transducer is a devicewhich is-preff erential to the passageof electromagnetic energy in one direction over passa e inthe reverse direction. When such a directional transducer is inserted in thetransmission line between the generator and the load, ,and preferablyplaced near the load, the efiect oi. slight changesQ-in'the load isconsiderably minimised because theimpedance thereafter presented to thetransmission line at its junction with the directional coupler newchanges only slightly with load changes.

It is an objectof the'present invention to-pro: vide a novel directionaltransducer.

It is a further object "of the inventiontoprovide such a transducerwhich "is of particularly simple structure.

It is a further object of the invention to provide a transmission linedirectional t-ransducereiima proved directional characteristics.

Figure 4 is a sectionalview of a directional transducer of the inventionin a rectangular waveguide in fwhich'the coupling" between the shuntsection and the, waveguide proper is less than unity;

Figure 5 is afacevieizv of adirectional trans.- ducer employing aplurality of directional trans-.- ducer sections of rectangularwaveguideaccording to theinvention'; I a

Figure 6 is a sectional view of a directional transducer of theinvention using a highly resistive impedance termination.

In accordance 'with the invention, my directional transducer comprises asection of transmission line; a reentrant transmission line portiion,and an impedan e mination. preferably.

energy absorbent, inserted in the reentrant portion." With :thetermination impedance of proper value and properly placed, the desireddirectional transducer and other desirable characteristics are secured,as will appear more fully hereinafter.

Referring now more particularly to Figure 1, there is shown a generatorI0, which may be a magnetron or any other type of ultra-high frequencygenerator. The generator may be matched to the line H through thetransducer by any of various well known means, not shown. The generator[0 is coupled to a load [2 through an-euectwww rc ransmissio li '4 and are ion l t ansduc r efh invention c isin he eciien lint tr s n wh h mayhe .cons de da or ien of th in l4. and 81sec? tion Qf'transrnission lineI8 reentrant with line ll andj s nt ith ero ien The l [2: may: also bematched to the line by any ct various well knownmeane. not shown. Theimnedanoe Z1 terminates the section It. at a point 4 intermediate to twojunctions with line H. The section, looking into the section in thedirection section [6 has a length a. The distance from in which thepassage of incident energy is the junction 20 between the transmissionline favored, appears to be a substantially stable load. section [8 andthe section H at the point of This may be stated still another wayqualitatively incident energy and the point at which imped- 5 by sayingthat whatever impedance may appear ance Z1 terminates the line sectionl8 hasalength on the remote side of the line, that side of the c. Thedistance between the point at which imline nearer the generator sees aload which tends pedance Z1 is shunted across the line section l8 toreceive energy but from which energy is reand the junction 22 of linesection l8 on the load fiected, if at all, in relatively constantamounts. side of line H is b. This results because the transducer tendsto pre- The directional transducer of Figure 1 may be vent the return ofenergy reflected from whatanalyzed by considering the impedance Zrlookever load may be placed on the remote side of ing into thetransducer arrangement at the juncthe directional transducer. tion 20,that is looking in the direction from the The action of the transducershown in Figure generator the loadh o wing Set Of l is not trulysusceptible of qualitative analysis.

equations may be derived by inspection of the The only proper approachto ascertain the action figure: thereof is byv inspection and study ofEquation 9. The new transducer presents to the line a EQUATIONS 1 to 8substantially constant impedance within moder- (1) ate limits ofvariation of the various parameters.

This is equivalent to saying that the relative variations in inputimpedance, Z'r, are less than the I 221 .b COS d-H S111 5 relativechanges in terminal impedance, Zn, and

' therefore the mis-matches and reflections due (3) to the undesiredchanges in Zn have less efiect upon the generator than they would havepro- I I I (4) duced without the network. Nevertheless, before I +1 17:1showing that such a result occurs, at least for m certain selectedvalues of the parameters in- E m= T= r 00$ B 5111 5 volved in theequation, we may qualitatively re- E ,1,=E,.=E, cos {ib+iI, Z sin 5b (7)view the structure of Figure 1 with a view to a better understanding ofits operation.

cos fic+dnzk Sm (8) The generator looks into a'line at the end of In theforegoing equations, the T5 are current which the line looks into animpedance. This e pressions, the Es are voltage eXD eS and impedance isthe impedance Zr of the equations 6 s t propagation constant Thus above.At-the junction 20 at which this imped- 27F ance is calculated, it willbe observed that the incident energy splits in two directions,neglecting for the moment any portion which may be where x is the linewavelength. All of the lines 40 reflected. One portion travels the linesection have a characteristic impedance Zk. The sub- Hi to the junction22 and the other portion scripts indicate the points at which thecurrent travels the line section 18, and only a part of and voltagevalues are taken, and are markedon this second portion reaches thejunction 22 be- I ,,=I, cos {Ba-H sin Ba Figure 1 in such a fashion asto be self-explanacause of the interposition of the line terminatory tothose skilled in the art. From this set of tion Z1. A momentsconsideration will convince equations it is possible to derive thefollowing eX- one that it is possible to choose the line lengthspression for Zr. and impedance Z1 in such a way that reflections fi s 1s [fi( )]i Z T: l

' 1 2 sin [B(b+c)] iZ b b S n[B(a+c)] Z tZ 00S ;3(bc)cos [B(b+c)] 2 SlIlflai 1 +c)] cos +C)]}+ iZ sin [fi(r+b)] Z sin fib sin 6b I sin Ba Z sin[B(b+c)] Z1 K eoS [fl( -i- 1 1'Z sin 8b sin Ba i sin [M i-11)] sin 8 Zi'Z sin Ba sin db 1 Z sin flb+ sin 6b iZ sin [[3(b+c)] Z l 1 sin[B(a+b)] sin dc Z 'iZ sin Ba sin Bb The various characteristics of thedirectional from the useful load are substantially or even transducersection may be analyzed by means entirely cancelled at the junction 20with reflecof Equation 9. Such analysis shows that this tions fromimpedance Z1, whereas energy from transducer section in general hasdirectional junction 20 is transmitted at junction 22 towardtransmissive characteristics. In other words, it the load. One oftheimportant characteristics preferentially transmits energy in onedirection of the new transducer arrangement, however, rather than in thereverse direction. The result cannot be explained in such a simplemanner. of such directional characteristics may also be Thischaracteristic is the insensitivity of Zr to expressed by saying thatthe directional coupler 7 changes in the impedance ZR.

perature variations. The other end of this quarter wavelength linesection is connected in Figure 1 to form the termination impedance Z1,so that I Z 22,, If ZR may change to (1+A)ZR Where A is con- Z 'iZK cosor T sin 2iZKZ1 cos a-Zfi sin (1 cos a Z Z (l-2 sin? a) 1,Z s1n a, Z cos04+ ZR cos ZR 2'LZ Z sin a I 2z'Z Z Z t-an a I v Z (23$in a) iZK -j-mZ Zs1n u+- R sin a It is clear, because of the periodicity of thefuncsiderably less than one, say less than rt, it may tions involvedthat [5a, oh, and 80 mayeach be be shown that p changed by amounts of21r without altering these I Z} results. Even with the assumption 1already T v made, there are still many possible values to choose whichmake ZT relatively constant-with changes in ZR. If ZT remains constantin this sense, at a value of ZR, then the standing wave ratio between agenerator and Zr is substantially independent of changes in ZR. Choose,for example, a as (ZN-1) 11', where N is an integer. Then Equation 10then reduces to:

Again, it will be understood that any of the quantities ca, 181), so maybe different by 21r from the above assumed values, without altering theresults.

The termination ZR may be considered as th impedance of the useful load.It will be understood that Za is subject to various changes, forexample, changes due to ambient temperature variations. Now, it isdesired that at least for small changes in ZR. the impedance Zr shallremain small. Thepresent structure has a further advantage. Not only isthe per unit change in ZT with respect to per unit changes in ZR, small,or even zero at selected values, but the second rate of change, of ZTwith respect to Zn is also small. This characteristic is even moreimportant than it is that the first rate of change be small. Theimportance of this advantage is that over a substantial range of valuesof ZR the rate of per unit change of Zn: with respect to per unitchanges in Za is small. In other words, the slope of the curve of ZTplotted with ZR asan independent variable is small and rem-ains'smallover a wide range of values. This range of values may readily be foundto the desired degree of accuracy and for the desired limiting values ofthe slope, by making the plot of Z: against ZR to scale. To calculate itis very difficult. When we say the slope is small, We mean that it isfractional, that is, less than one. 1

Let

be the characteristic impedance of a section ofa quarter wavelengthtransmission line atone end of which there is a termination havinganimpedance substantially equal to the lo-ad impedance ZR and undergoingsubstantially the same changes, for example, those due to ambienttemapproximately, which indicates that the change in Zr'is only about45% as great as that in .Za.

As a second example if Z1 is madeof a material like barium titanatewhich has a decreasing re-' sistivity with field strength, then bysuitable choice of ZK, Z121, and Z1 it is again possible to reduce thevariation of Z'r with changing ZR.

For Z1:Za,. and assuming that Zn changes to ZR(1+A) where A isconsiderably less than one, then it may be shown'that which indicates achange in Zr only about 37% as great as the. change in ZR. Thus it willbe observed that with such disparate values as Zl=ZK /2ZR and Z1:Za, forthe two conditions, the difference in the rate of change of Zr with ZRvariesonly by about 8%.

Referring now to Figure 2, which is a sectional view of a directionaltransducer of the invention, a quarter wavelength section 30 of hollowpipe rectangular cross-section transmission line is shuntedby a section32 of similar hollow pipe line. The section 32 of transmission line isreentrant on the section 30 having junctions therewith at 34 and 36. Aline termination 38 is inserted in the section 32 at a distance an oddnumber of half wavelengths from the junction 36 and the same odd numberof half wavelengths plus a quarter wavelength distant from the junction34.

Thus the length 'of the entire section 32 is twice an odd number of halfwavelengths plus a quarter wavelength, as may be the case for thespecific. values chosen for Figure 1 to derive Equation 10. Since ca, 81and Bo occur in periodic functions with a periodicity of Z any of orthat side of the line transmission which is to be favored. A similardirectional transducer is shown in Figure 3 in face view, in which thewaveguide transmission line is cylindrical in cross-section and theconstruction and nature 7 thereof will be obvious to those skilled inthe art from what has been said heretofore.

It will be observed that in Figures 2 and 3 the coupling of thereentrant section is effected y utilizing a complete opening formed atthe intersection of the internal surfaces of the line section and thereentrant section at the junctions. A somewhat different arrangement isshown in Figure 4 illustrating a directional transducer of the inventionfor use with rectangular Waveguides. In Figure 4 the junctions are at 50and 52. The section of the main line is 54 and the reentrant section is56. The termination in line 55 is shown at 58 as a waveguide stub,similar to the stubs utilized for such terminations in Figures 2 and 3.However, at all of the junctions shown in Figure 4, the couplingbetween, for example, the reentrant section 56 and the line section 54at 59 and 52, are formed by windows in section 54 allowing only limitedcommunication at the junction between section 54 and the reentrantsection 56. I often prefer the use of such window couplings, becausethey reduce the coefficient of coupling between the line proper and thereentrant section and thus tend to avoid reflections back in thedirection from which the incident energy arrives. The directionaltransducers of Figures 2, 3 and 4 may be analyzed by the same methodsemployed in the analysis of the coupler of Figure 1, and with thesimilar results. The appropriate application of these formula to thewaveguide structures will be understood by those skilled in the art fromwhat has been said heretofore.

A preferred embodiment of the invention is that illustrated in Figure 5which includes a plurality (three of which are shown) of suchdirectional transducers as those shown in Figure 4 placed in tandemalong the line. Directional effects are enhanced by using a plurality ofthe directional transducers of Figure 4 arranged as shown in Figure 5,each of which contributes to the directional transducer effect and yet,because of the small coefficient of coupling with the reentrantportions, reflections from "the transducer of the incident energy backin the direction from which it arrives are reduced. With a plurality ofsuch sections, any change in impedance looking into the first sectiondue to the change in a load on the end of the other section will bereduced as the product of the fractional change, at least for very smallchanges to a first approximation, which would be resultant from onealone; thus supposing that for each of two such sections, the change inZr is about /2 for a given change in ZR in the notation of Figure 1.When combined the change in Zr of the two sections will be about A; forthe same change in the load. Lossy material may be included in theterminations 59, as shown.

Figure 6 shows a directional transducer of the invention which isinserted in a coaxial line. The termination impedance Z'r in thisinstance comprises a resonantcavity 60 coupled to the reentrant branch62 by a line stub 64. The cavity is coupled to the line stub 64 by anexciting probe 66.v The cavity is filled with an energy absorbentmaterial 68 to increase the resistive component to a desired value. Ahigh resistive component is generally desirable in the terminationimpedance Zr. Coupling may be further reduced by removing a part of thecontacting portions 62a and 62b to provide capacitive coupling of theshunt section.

What is claimed is:

1. A linear passive network directional transducer comprising atransmission line section substantially a quarter wavelength long at theoperating frequency, a reentrant transmission line section havingjunctions with said quarter Wavelength section at the ends thereof, animpedance termination in said reentrant section at a point an odd numberof half wavelengths from one said junction and an odd number of halfwavelengths plus a quarter wavelength distant from the other saidjunction, said impedance termination terminating the portions of saidreentrant line section on each side thereof in a, highly resistive powerabsorbing impedance having a value different substantially from that ofthe characteristic impedance thereof.

2. A linear passive network directional transducer comprising a firsttransmission line section having a length \/4+(n11) at the operatingfrequency, a reentrant transmission line section having junctions withsaid first section at the ends thereof, and a highly resistive powerabsorbing impedance termination in said reentrant section at a point(2nz1) \/2 from one said junction and (2n31) \/2+ \/4 from the othersaid junction, n1, m and n3 each being a positive whole number, A beingthe line wavelength at the frequency of operation the impedance of saidtermination being substantially different from the characteristicimpedance of said reentrant section.

3. A linear passive network directional transducer comprising a firsttransmission line section having a length \/4+(n1-1) at the operatingfrequency, a reentrant transmission line section having junctions withsaid first section at the ends thereof, and an impedance termination insaid reentrant section at a point (2n-1) \/2 from one said junction andfrom the other said junction, m, m and 11,3 each being a positive wholenumber, i being the line wavelength at the frequency of operation, saidtermination impedance including a highly resistive power absorbingcomponent and differing substantially in value from the linecharacteristic impedance.

4. The directional transducer claimed in claim 2, wherein saidtransmission line sections are hollow pipe wave guides.

5. The directional transducer claimed in claim 2 wherein saidtransmission line sections are two-conductor transmission line sections.

6. The directional transducer claimed in claim 1, the coupling betweenthe sections at said junctions being very much less than unity coupling.

7. The combination comprising a plurality of transducers as claimed inclaim 1, connected in tandem.

8. The combination comprising a plurality of transducers as claimed inclaim 2, connected in tandem.

LOWELL E. NORTON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,147,809 Alford Feb. 21, 1939 2,190,131 Alford Feb. 13, 19402,226,686 Alford Dec. 31, 1940 2,251,997 Goldmann Aug. 12, 19412,519,734 Bethe Aug. 22, 1950

